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While it's easier to spot the difference between two faces that are similar if they're orientate the right way up, the same isn't true if they're upside down.

'This is because we have a specific brain area for processing faces, and when the face is upside down, we process the image through object processing pathways, and not the face-processing pathways any more,' Dr Mu-Yun Wang, a researcher at the University of Tokyo and the co-author of the study, told New Scientist.

Humans read the face as a whole, rather than look at parts of the face, which is why it's harder to recognize a face when it's upside-down.

Researchers used to think that this effect only occurred in mammals, but now it's also been observed in the Japanese rice fish - also called the medaka (Oryzias latipe).

HOW THEY DID THE EXPERIMENT

To find out how medaka fish identify each others, Dr Wang and his co-author Dr Hideaki Takeuchi conducted an experiment where the first allowed a female to familiarize herself with a male, because females tend to mate faster with familiar males.

Then, the researchers disguised the male in various ways.

First, the used a near transparent film to mask either the face, body or tail to see if the female would still recognize the male.

It was only when the male's face was covered that the female reacted in a way that suggested she didn't recognize the fish.

To see how well medaka recognize upside-down faces, the researchers used a prism to invert the face of a male vertically (right) or horizontally (left), and tested how the female reacted in both cases

The researchers also painted a pattern on a male medaka’s face or tail using a black marker, and the fish could still recognize each other.

To see how well medaka recognize upside-down faces, the researchers used a prism to invert the face of a male vertically or horizontally, and tested how the female reacted in both cases.

Native to East Asia, the Japanese rice fish reaches about 3.2 centimeters in length and is commonly found in ponds, paddy fields and in the small flows of plain.

According to the researchers, previous research has shown that some fish species can distinguish between individuals.

The researchers also painted a pattern on a male medaka’s face or tail using a black marker, and the fish could still recognize each other

For example, female medaka fish prefer males they have seen before to strangers, but until known, researchers didn't know they could recognize individual faces, nor how they distinguish a specific male from many others.

To find out how medaka fish identify each others, Dr Wang and his co-author Dr Hideaki Takeuchi conducted an experiment where the first allowed a female to familiarize herself with a male, because females tend to mate faster with familiar males.

Then, the researchers disguised the male in various ways.

First, the used a near transparent film to mask either the face, body or tail to see if the female would still recognize the male.

It was only when the male's face was covered that the female reacted in a way that suggested she didn't recognize the fish.

The researchers also painted a pattern on a male medaka’s face or tail using a black marker, and the fish could still recognize each other.

Humans read the face as a whole, rather than look at parts of the face, which is why it's harder to recognize a face when it's upside-down. Researchers used to think that this effect only occurred in mammals, but now it's been observed in the Japanese rice fish (Oryzias latipe). Pictured is a photoshopped, vertically inverted Japanese rice fish facing another

To see how well medaka recognize upside-down faces, the researchers used a prism to invert the face of a male vertically or horizontally, and tested how the female reacted in both cases.

'It appears that in the process of evolving specialized face-recognition abilities to quickly and accurately extract important information, there has been a trade-off where face-like images in unexpected orientations become especially difficult to process,' Dr Michael Sheehan, a behavioral ecology researchers at Cornell University, told New Scientist.